Take time to work out what each costs per hour. (You will need your gas and electricity bills)
According to my electric bill the cost per kWh (kilowatt hour) is .05801 plus .03691 for delivery charges
The space heater uses 4,700 BTUs which calculates at 1,377 per kWh using the calculator on Rapid Tables
Natural Gas shows .58634 per CCF (hundred cubic feet) on my bill (this includes a Rider charge). To calculate the BTUs (British Thermal Units) used by your furnace take a look at your manual or look online for approximate BTUs for your type of furnace.
To calculate gas consumption and cost, review The Garden Web instructions
My calculations show that for me, using a space heater is half the cost of turning on the furnace, but by plugging in two or more it would actually cost the same or more than using the gas furnace.